Assuming the following HMAC is used :
HMACK(M) = Hash(K||M) = H which is "secret-prefix" hashed function. M is the plaintext message , K is the secret key used
The ciphertext is generated as follow :
Ciphertext = EK(M||H) where EK is a encryption scheme which uses the secret Key K.
I have read on Wikipedia that a length extension attacks works on hashes where the secret key length and message is known. In this case as I have encrypted the message , I would like to confirm that it is no longer possible to conduct a length extension attack as M is unknown because I have encrypted it.
I am also like to know if the above scheme used to authenticate messages is susceptible to any other attacks ??